Fluorine-free cathode binder, cathode sheet, and lithium ion battery

Abstract

This application provides a fluorine-free positive electrode binder, a positive electrode sheet, and a lithium-ion battery. The fluorine-free positive electrode binder comprises a copolymer, which includes linear polysaccharide structural units, thiocarbonyl sulfide structural units, acrylonitrile structural units, and acrylate structural units. The molar number of the thiocarbonyl sulfide structural units is N1, and the total molar number of the acrylonitrile and acrylate structural units is N2, with a N1:N2 ratio of 1:(25~35). This fluorine-free positive electrode binder integrates the functions of "fluorine-free compliance, flexibility, bonding, stable connection, and ion conduction," and is compatible with existing NMP solvent systems. It is suitable for mainstream positive electrode active materials and, when applied to positive electrode sheets, simultaneously enhances flexibility and interfacial stability, resulting in structural stability and excellent cycle life in the battery.

Description

Technical Field

[0001] This application belongs to the field of battery materials technology, specifically relating to a fluorine-free positive electrode binder, a positive electrode sheet, and a lithium-ion battery. Background Technology

[0002] With the explosive growth in global demand for renewable energy and electric vehicles, developing lithium-ion batteries with higher energy density, longer cycle life, and better environmental performance has become a core focus of scientific research and industrial development. As the core component of lithium-ion batteries, the performance of the cathode material directly determines the battery's energy density and cycle stability.

[0003] Currently, the mainstream active materials for the positive electrode of lithium-ion batteries, such as lithium iron phosphate (LFP), lithium nickel cobalt manganese oxide (NMC), and lithium nickel cobalt aluminum oxide (NCA), typically use polyvinylidene fluoride (PVDF) or polyacrylonitrile (PAN) as binders. After being dissolved in NMP solvent, they are mixed with conductive agents to form a slurry, which is then coated onto an aluminum foil current collector. After drying and pressing, the positive electrode sheet is prepared and then used to manufacture lithium batteries.

[0004] Although these cathode materials exhibit relatively mild volume changes (2-6%) during charge and discharge, they still face key challenges such as interfacial bonding failure and environmental compliance. Specifically, PVDF, as a fluorinated binder, not only suffers from weak interfacial bonding with the cathode active material and conductive agent due to its reliance solely on van der Waals forces, making it difficult to buffer the volume changes of the cathode material, but also prone to active material shedding during long-term cycling, leading to the collapse of the conductive network. Furthermore, due to the serious fluorine pollution throughout its entire life cycle, it faces increasingly stringent restrictions from regulations such as the EU REACH, resulting in extremely high compliance risks. While PAN provides high bonding strength by binding with the cathode active material particles through the polar effect of cyano groups and is fluorine-free and compliant, its rigid molecular chains and extremely low degree of freedom of chain segment movement make it unable to buffer the stress generated by volume changes. It also has a low elongation at break (approximately 20%), is hard and brittle, and is prone to cracking. It cannot effectively adapt to the volume changes of the cathode material, and is prone to electrode cracking due to stress concentration during long-term cycling, resulting in poor cycle stability.

[0005] In summary, current mainstream binders generally cannot simultaneously achieve the multiple functions of "fluorine-free compliance, flexible buffering, and strong adhesion." This imbalance between performance and compliance has become a key bottleneck restricting the industrialization of high energy density, long cycle life, and green cathode materials. Therefore, there is an urgent need in this field for a novel cathode binder solution that can integrate strong adhesion, fluorine-free environmental friendliness, good flexibility, and compatibility with existing processes. Summary of the Invention

[0006] To address the problems existing in current cathode binders, this application provides a fluorine-free cathode binder, cathode sheet, and lithium-ion battery.

[0007] The purpose of this application is to achieve the following technical solution.

[0008] In a first aspect, this application provides a fluorine-free positive electrode binder, the fluorine-free positive electrode binder comprising a copolymer, the copolymer comprising linear polysaccharide structural units, sulfocarbonyl thio groups, acrylonitrile structural units, and acrylate structural units, wherein the molar number of the sulfocarbonyl thio groups is N1, the total molar number of the acrylonitrile structural units and the acrylate structural units is N2, and the N1:N2 ratio is 1:(25~35).

[0009] Furthermore, the electrolyte swelling ratio of the fluorine-free positive electrode binder is less than 6%.

[0010] Furthermore, the ratio of the total mass of the linear polysaccharide structural unit to the total mass of the structural unit containing the thiocarbonyl thio group is 100:(3~7).

[0011] Furthermore, the molar ratio of the acrylonitrile structural unit to the acrylate structural unit is 7:(2~4).

[0012] Furthermore, the linear polysaccharide structural unit is at least one of the following: natural cellulose structural unit, microcrystalline cellulose structural unit, nanocellulose structural unit, and chitin structural unit.

[0013] Furthermore, the structural unit containing the thiocarbonyl thio group is derived from the RAFT reagent containing the carboxyl group.

[0014] Furthermore, the acrylate structural unit is selected from at least one of polyethylene glycol acrylate structural units, methoxy polyethylene glycol acrylate structural units, methoxy polyethylene glycol methacrylate structural units, and ethylene glycol methyl ether acrylate structural units.

[0015] Furthermore, the copolymer also includes phosphate ester structural units, and the molar ratio of the acrylonitrile structural units to the phosphate ester structural units is 7:(0.3~3).

[0016] Secondly, this application provides a positive electrode sheet, said positive electrode sheet comprising the fluorine-free positive electrode binder as described in the first aspect.

[0017] Thirdly, this application provides a lithium-ion battery, which includes the fluorine-free positive electrode binder described in the first aspect or the positive electrode sheet described in the second aspect.

[0018] Compared with the prior art, this application has the following beneficial effects.

[0019] 1) Fluorine-free, environmentally compliant, and process-compatible: The fluorine-free cathode binder in this application does not contain fluorine, completely eliminating fluorine pollution in the production and disposal stages of PVDF from the source, making it environmentally compliant and meeting EU environmental regulations. At the same time, it is compatible with mainstream cathode active materials such as LFP, NMC, and NCA, and is compatible with existing NMP solvent systems and cathode coating equipment, eliminating the need to modify production lines, reducing industrialization costs, and resolving the contradiction between environmental compliance and process compatibility.

[0020] 2) Simultaneous Enhancement of Flexibility and Interfacial Stability: By introducing acrylate flexible segments, the rigid structure of the linear polysaccharide structural units is effectively disrupted, giving the binder network excellent deformation recovery ability. This buffers the volume change of the positive electrode active material, resulting in superior flexibility compared to PAN and PVDF. Acrylonitrile structural units, acting as bonding reinforcement units, provide strongly polar cyano groups that bond with the particle surface of the positive electrode active material (such as the PO4 group of LFP). 3- The oxygen atoms of NMC form dipole-dipole interactions and hydrogen bonds, which increases the interfacial bonding force between the binder and the active material and aluminum current collector. The interfacial bonding force is better than that of PAN and PVDF. During battery cycling, there is no active material shedding or cracking, and the integrity of the electrode structure is maintained.

[0021] 3) Excellent structural stability and cycle life, with improved rate performance: The linear polysaccharide structural units and the sulfur-containing carbonyl sulfide structural units form a stable connection. Simultaneously, the controlled polymerization of the sulfur-containing carbonyl sulfide structural units achieves precise and regular grafting of acrylonitrile and acrylate structural units, ensuring strong bonding between each structural unit and preventing unit detachment during cycling, thus improving network structural stability. The fluorine-free structure avoids electrolyte decomposition caused by fluorine, and the rigid network of linear polysaccharide and acrylonitrile structural units is broken, reducing interfacial stress concentration and preventing electrode cracking due to stress concentration during long-term cycling. The acrylate structural units, as flexible and ion-conducting units, not only improve the flexibility of the binder but also ensure good solubility and lithium-ion conductivity of the binder in NMP. The binder of this application exhibits excellent cycle stability and high rate performance during battery cycling, with overall performance superior to PVDF-based and PAN-based cathodes. Detailed Implementation

[0022] To make the technical problems, technical solutions, and beneficial effects solved by this application clearer, the following detailed description is provided in conjunction with specific embodiments. It should be understood that the embodiments described herein are only a part of the embodiments of this application, not all of them, and are merely used to explain this application and are not intended to limit it. All other embodiments obtained by those skilled in the art based on the embodiments in this application without inventive effort are within the protection scope of this application.

[0023] It should be noted that, in this application, as is known to those skilled in the art of chemical synthesis, each structural unit represents the structural portion of the corresponding monomer present in the resulting polymer after the monomer participates in the polymerization reaction. The mass ratio of each structural unit is the mass ratio of the monomers providing each structural unit.

[0024] In a first aspect, this application provides a fluorine-free positive electrode binder, the fluorine-free positive electrode binder comprising a copolymer, the copolymer comprising linear polysaccharide structural units, sulfocarbonyl thio groups, acrylonitrile structural units, and acrylate structural units, wherein the molar number of the sulfocarbonyl thio groups is N1, the total molar number of the acrylonitrile structural units and the acrylate structural units is N2, and the N1:N2 ratio is 1:(25~35).

[0025] The copolymer comprises a main chain and side chains, with linear polysaccharide structural units as the main chain, and acrylonitrile structural units and acrylate structural units polymerized and grafted onto the side chains.

[0026] After grafting modification, linear polysaccharides retain the abundant hydroxyl groups in their polysaccharide backbone, enabling them to bind with the surface of positive electrode active materials (such as PO4 in LFP). 3- The oxygen atoms of NMC form strong hydrogen bonds. Simultaneously, the grafting of flexible acrylate monomers disrupts the rigid crystal structure of the main chain and the dense hydrogen bond network between the main chains. This, combined with the flexible side chains, enhances the chain segment mobility of the resulting composite material, resulting in excellent overall flexibility. The cyano group (-CN) of the acrylonitrile structural unit has extremely strong polarity, which can strengthen the interfacial interaction between the binder and the cathode particles; the acrylate structural unit combines excellent flexibility and ion conductivity, further improving the deformation buffering capacity of the binder and the lithium-ion conduction efficiency.

[0027] Specifically, N1:N2 can be 1:25, 1:26, 1:27, 1:28, 1:29, 1:30, 1:32, 1:33 or 1:35, etc.

[0028] In some specific embodiments, the linear polysaccharide structural unit is derived from a linear polysaccharide, and the structural unit containing a thiocarbonyl sulfide group is derived from a carboxyl-containing RAFT reagent, wherein the RAFT reagent contains a thiocarbonyl sulfide group.

[0029] In some specific embodiments, the linear polysaccharide structural unit is connected to the thiocarbonyl thio group-containing structural unit by an ester bond, which is formed by the esterification reaction of the linear polysaccharide with a carboxyl-containing RAFT reagent.

[0030] Linear polysaccharides, with their rigid, fluorine-free backbone, provide deformation buffering and esterification grafting sites. Carboxyl-containing RAFT reagents serve as esterification linking units and chain transfer agents, enabling covalent bonding of the main and side chains and precise monomer grafting. The hydroxyl groups of the linear polysaccharides and the carboxyl-containing RAFT reagents form stable ester bonds through esterification, resulting in strong covalent connections and replacing traditional physical blending methods. Simultaneously, the carboxyl-containing RAFT reagents act as chain transfer agents, enabling precise grafting of acrylonitrile and acrylates, avoiding side chain loss and uneven distribution, and improving the long-term stability of the network structure.

[0031] By constructing a fluorine-free integrated network through RAFT controlled polymerization technology, it is possible to achieve the functional synergy of linear polysaccharide structural unit main chain providing structural rigidity (strength) and environmental compliance, acrylonitrile structural unit providing bonding strength, acrylate structural unit enhancing flexibility and ion conduction, and RAFT reagent achieving stable connection of main and side chains. This fundamentally solves the problems of performance imbalance, fluorine pollution and compliance risks of existing cathode binders, while also being compatible with the industrial production process of existing NMP solvent systems.

[0032] In some specific embodiments, the linear polysaccharide structural unit is at least one selected from natural cellulose structural units, microcrystalline cellulose structural units, nanocellulose structural units, and chitin structural units. The linear polysaccharide is at least one selected from natural cellulose, microcrystalline cellulose, nanocellulose, and chitin. All linear polysaccharides contain hydroxyl structures that can provide active reaction sites and possess fluorine-free and environmentally friendly properties.

[0033] The carboxyl-containing RAFT reagent is at least one selected from 4-cyano-4-((dodecylthiocarbonyl)thio)valerate (CAS: 870196-80-8), 2-(((dodecylthio)thiocarbonyl)thio)propionic acid (CAS: 558484-21-2), bis(carboxymethyl)trithiocarbonate (CAS: 6326-83-6), and 2-(dodecylthiothiocarbonylthio)-2-methylpropionic acid (CAS: 461642-78-4). The carboxyl-containing RAFT reagent of this application can undergo esterification with the hydroxyl groups of linear polysaccharides and exhibits excellent chain transfer properties.

[0034] In some specific embodiments, the mass ratio of the linear polysaccharide structural unit to the thiocarbonyl thio group-containing structural unit is 100:(3~7), which is the mass ratio of the linear polysaccharide structural unit to the thiocarbonyl thio group-containing structural unit during feeding. Specifically, the mass ratio of the linear polysaccharide structural unit to the thiocarbonyl thio group-containing structural unit can be 100:3, 100:3.5, 100:4, 100:4.5, 100:5, 100:5.5, 100:6, 100:6.5, or 100:7, etc.

[0035] In some specific embodiments, the molar ratio of the acrylonitrile structural unit to the acrylate structural unit is 7:(2~4). The molar ratio of the acrylonitrile structural unit to the acrylate structural unit can be 7:2, 7:2.3, 7:2.9, 7:3, 7:3.3, 7:3.6, or 7:4, etc. Using this ratio achieves an optimal balance between bond strength, flexibility, and ionic conductivity (rate performance) while ensuring fluorine-free compliance, flexibility, and NMP solubility.

[0036] In some specific embodiments, the acrylonitrile structural unit is an acrylonitrile structural unit or a methacrylonitrile structural unit, derived from acrylonitrile monomers, namely acrylonitrile or methacrylonitrile, respectively. The cyano groups (-CN) of acrylonitrile and methacrylonitrile have similar polarities and matched copolymerization activities, achieving equivalent bond strength. The cyano group can bond with the particle surface of the positive electrode active material (such as PO4 in LFP). 3- (Oxygen atoms in NMC, etc.) form dipole-dipole interactions and hydrogen bonds, increasing the interfacial bonding force between the binder and the active material and aluminum current collector.

[0037] In some specific embodiments, the acrylate structural units are selected from at least one of polyethylene glycol acrylate structural units, methoxy polyethylene glycol acrylate structural units, methoxy polyethylene glycol methacrylate structural units, and ethylene glycol methyl ether acrylate structural units. The acrylate structural units are derived from acrylate monomers, which are selected from at least one of polyethylene glycol acrylate (PEGMA), methoxy polyethylene glycol acrylate, methoxy polyethylene glycol methacrylate, and ethylene glycol methyl ether acrylate with different molecular weights. Acrylate monomers not only improve the flexibility and NMP solubility of the binder, but also provide a fast conduction channel for lithium ions through the ether bonds in their structure, reducing battery internal resistance and exhibiting good lithium-ion conductivity.

[0038] Polyethylene glycol acrylate exhibits superior ion conductivity. With its polyether chain backbone containing numerous ether bonds, ethylene glycol acrylate is the core component for lithium-ion conduction. Lithium ions coordinate / complex with the oxygen atoms of these ether bonds. Through the segmental movement of the polymer chain, lithium ions continuously undergo a "jump-like" migration process of "complexation-dissociation-re-complexation" with intra- or inter-chain ether oxygen atoms, providing a rapid migration path with low energy barriers for lithium ions. This significantly improves lithium-ion conductivity and, consequently, enhances the rate performance of the battery.

[0039] In some specific embodiments, the copolymer further includes phosphate ester structural units, wherein the molar ratio of the acrylonitrile structural units to the phosphate ester structural units is 7:(0.3~3). The phosphate ester structural units are derived from phosphate methacrylate. These phosphate ester structural units can form stronger coordination bonds with the surface of the positive electrode active material, further improving interfacial adhesion and electrolyte tolerance. Simultaneously, the phosphate ester can help improve lithium-ion conductivity and optimize the high-rate performance of the battery.

[0040] The fluorine-free positive electrode binder of this application is based on a linear polysaccharide as the main chain. First, a linear polysaccharide-RAFT reagent is prepared by esterification reaction with a carboxyl-containing RAFT reagent. Then, the linear polysaccharide-RAFT reagent is used as a chain transfer agent to carry out polymerization and grafting reaction with acrylonitrile and acrylate monomers to obtain a copolymer. After purification and molding, a fluorine-free positive electrode binder containing the above copolymer is obtained.

[0041] The fluorine-free cathode binder of this application can be obtained using conventional esterification and RAFT polymerization grafting reactions in the art. However, by using the preparation conditions and steps of this application, the esterification reaction can be ensured to proceed fully, the density of linear polysaccharide-RAFT reagent grafting sites can be optimized, and the precise and regular grafting of monomers can be achieved, ensuring the structural stability and performance consistency of the binder. Specifically, it can be prepared according to the following steps: The first step is the preparation of the linear polysaccharide-RAFT reagent: The linear polysaccharide is added to a solvent according to the specified mass ratio, stirred at 80°C for 2 hours to disperse it evenly, cooled to 40°C, and then a carboxyl-containing RAFT reagent and an appropriate amount of esterification catalyst are added. The mixture is stirred at 40°C for 14 hours under a nitrogen atmosphere to induce an esterification reaction, yielding a solution of the linear polysaccharide-RAFT reagent. The preferred solvent is N-methylpyrrolidone (NMP); the esterification catalyst is a DCC / DMAP complex, wherein the mass ratio of DCC (dicyclohexylcarbodiimide) to DMAP (4-dimethylaminopyridine) is 3:1. Alternatively, a 1-ethyl-(3-dimethylaminopropyl)carbodiimide hydrochloride (EDC·HCl) / N-hydroxysuccinimide (NHS) complex can be used as the esterification catalyst, exhibiting comparable catalytic efficiency and easier removal of byproducts, making it more environmentally friendly.

[0042] The second step is RAFT controlled graft polymerization: Acrylonitrile monomers, acrylate monomers, and an initiator are added to the solution of linear polysaccharide-RAFT reagent according to the molar ratio. Using linear polysaccharide-RAFT reagent as a chain transfer agent, the polymerization grafting reaction is carried out under a nitrogen atmosphere at 65~75℃ for 8~10h to obtain a copolymer. The initiator is azobisisobutyronitrile (AIBN), and its addition amount is 0.8~1.2% of the total mass of monomers.

[0043] The third step is purification and shaping: After the reaction is completed, the mixture is cooled to room temperature, or precipitated with anhydrous ethanol and dried under vacuum at 60°C for 12 hours to obtain a solid binder, which is the fluorine-free positive electrode binder of this application.

[0044] Secondly, this application provides a positive electrode sheet, said positive electrode sheet comprising the fluorine-free positive electrode binder as described in the first aspect.

[0045] The fluorine-free positive electrode binder of this application is a solid. When used, it is dissolved in NMP solvent to prepare an NMP solution with a solid content of 3~5wt%. The fluorine-free positive electrode binder is mixed with the positive electrode active material and conductive agent at a mass ratio of (1~5):(90~96):(1~5) to form a slurry, which is then coated onto an aluminum current collector. After being dried with hot air at 80℃ for 1h, it is dried under vacuum at 120℃ for 12h. After being pressed into a sheet at 10~15MPa, a positive electrode sheet is obtained.

[0046] The "solid content" mentioned above refers to the percentage of the mass of components other than the solvent in the NMP solution relative to the total mass of the NMP solution.

[0047] The fluorine-free cathode binder of this application can be used to prepare cathode sheets and is compatible with cathode active materials such as lithium iron phosphate (LFP), lithium nickel cobalt manganese oxide (NMC), and lithium nickel cobalt aluminum oxide (NCA). The conductive agent includes, but is not limited to, at least one of conductive carbon black (Super P), conductive graphite, Ketjen black, acetylene black, carbon nanotubes, carbon fibers, graphene, and conductive polymers.

[0048] The mass ratio of fluorine-free positive electrode binder to positive electrode active material and conductive agent is (1~3):(94~96):(2~5). Specifically, when adapted to LFP positive electrode active material, the mass ratio of fluorine-free positive electrode binder to positive electrode active material and conductive agent is 2:95:3; when adapted to NMC or NCA positive electrode active material, the mass ratio is 2.5:95:2.5. This maximizes the proportion of active material while ensuring the conductivity and structural integrity of the electrode, adapting to the performance requirements of different positive electrode materials.

[0049] Thirdly, this application provides a lithium-ion battery, which includes the fluorine-free positive electrode binder described in the first aspect or the positive electrode sheet described in the second aspect.

[0050] The specific implementation methods of this application will be further explained and illustrated below through examples and comparative examples.

[0051] Unless otherwise specified, all reagents, materials, and instruments used in the following description are conventional reagents, materials, and instruments, all of which are commercially available. The reagents involved can also be synthesized using conventional synthetic methods. Unless otherwise specified, the methods in the examples are conventional methods in the art. Monomers conforming to this application are commercially available.

[0052] Example 1 1) Preparation of fluorine-free positive electrode binder: Natural cellulose was added to the solvent NMP and stirred at 80°C for 2 hours to disperse it evenly. After cooling to 40°C, 4-cyano-4-((dodecylthiocarbonyl)thio)valerate and an appropriate amount of DCC / DMAP complex were added. The mass ratio of natural cellulose to 4-cyano-4-((dodecylthiocarbonyl)thio)valerate was 100:5. The mixture was stirred at 40°C for 14 hours under a nitrogen atmosphere to induce an esterification reaction, thus obtaining a solution of linear polysaccharide-RAFT reagent. Acrylonitrile, polyethylene glycol acrylate, and azobisisobutyronitrile were added to a solution of linear polysaccharide-RAFT reagent. The molar ratio of 4-cyano-4-((dodecylthiocarbonyl)thio)valerate to the total molar ratio of acrylonitrile and polyethylene glycol acrylate was 1:30, the molar ratio of acrylonitrile to polyethylene glycol acrylate was 7:3, and the amount of azobisisobutyronitrile added was 1% of the total monomer mass. Using linear polysaccharide-RAFT reagent as a chain transfer agent, a polymerization grafting reaction was carried out under a nitrogen atmosphere at 68°C for 9 hours to obtain a copolymer. After the reaction, the copolymer was precipitated with anhydrous ethanol and dried under vacuum at 60°C for 12 hours to obtain a solid binder, which is the fluorine-free positive electrode binder of this application.

[0053] 2) Preparation of the positive electrode: The prepared fluorine-free positive electrode binder was dissolved in NMP solvent to prepare an NMP solution with a solid content of 4wt%. The fluorine-free positive electrode binder was mixed with lithium iron phosphate (LFP) and conductive carbon black (Super P) at a mass ratio of 2:95:3 to form a slurry. The slurry was coated on the surface of the positive electrode current collector aluminum foil, dried with hot air at 80℃ for 1 hour, and then vacuum dried at 120℃ for 12 hours. After pressing at 12MPa, the positive electrode sheet was prepared.

[0054] 3) Preparation of lithium-ion batteries: The prepared positive electrode sheet, separator, and negative electrode sheet are stacked in sequence, with the separator positioned between the positive and negative electrodes to act as an separator, thus forming an electrode assembly. The electrode assembly is placed in an outer package, injected with commercially available electrolyte, and sealed. After processes such as electrolyte injection, formation, and degassing, a lithium-ion battery is obtained.

[0055] Example 2 This embodiment uses most of the operating steps described in Example 1 to prepare a fluorine-free positive electrode binder, positive electrode sheet, and lithium-ion battery. The difference lies in the raw materials used to prepare the fluorine-free positive electrode binder: in addition to acrylonitrile and polyethylene glycol acrylate monomers, phosphate methacrylate is added to the linear polysaccharide-RAFT reagent solution, with a molar ratio of acrylonitrile to phosphate methacrylate of 7:1. The rest is the same as in Example 1.

[0056] Example 3 This embodiment uses most of the operating steps described in Example 1 to prepare a fluorine-free positive electrode binder, positive electrode sheet, and lithium-ion battery. The difference lies in the raw materials used to prepare the fluorine-free positive electrode binder: the carboxyl-containing RAFT reagent is replaced with 2-(((dodecylthio)thiocarbonyl)thio)propionic acid, and the acrylonitrile monomer is replaced with methacrylonitrile. The rest is the same as in Example 1.

[0057] Example 4 This embodiment uses most of the operating steps described in Example 1 to prepare a fluorine-free positive electrode binder, positive electrode sheet, and lithium-ion battery. The difference lies in the raw materials used to prepare the fluorine-free positive electrode binder: the RAFT reagent is replaced with bis(carboxymethyl)trithiocarbonate, and the acrylate monomer is replaced with methoxy polyethylene glycol methacrylate. The rest is the same as in Example 1.

[0058] Example 5 This embodiment uses most of the operating steps described in Example 1 to prepare a fluorine-free positive electrode binder, positive electrode sheet, and lithium-ion battery. The difference lies in the raw materials used to prepare the fluorine-free positive electrode binder: linear polysaccharides are replaced with chitosan, acrylonitrile monomers are replaced with methacrylonitrile, and acrylate monomers are replaced with methoxy polyethylene glycol acrylate. The rest is the same as in Example 1.

[0059] Example 6 This embodiment uses most of the operating steps described in Example 1 to prepare a fluorine-free positive electrode binder, positive electrode sheet, and lithium-ion battery. The difference lies in the raw materials used to prepare the fluorine-free positive electrode binder: linear polysaccharides are replaced with chitosan, the RAFT reagent is replaced with 2-(((dodecylthio)thiocarbonyl)thio)propionic acid, acrylonitrile monomers are replaced with methacrylonitrile, and acrylate monomers are replaced with ethylene glycol methyl ether acrylate. The rest is the same as in Example 1.

[0060] Example 7 This embodiment uses most of the operating steps in Example 1 to prepare a fluorine-free positive electrode binder, positive electrode sheet, and lithium-ion battery. The difference lies in the amount of raw materials used in the preparation of the fluorine-free positive electrode binder: the mass ratio of natural cellulose to 4-cyano-4-((dodecylthiocarbonyl)thio)valerate is 100:3, the molar ratio of 4-cyano-4-((dodecylthiocarbonyl)thio)valerate to acrylonitrile and polyethylene glycol acrylate is 1:25, and the molar ratio of acrylonitrile to polyethylene glycol acrylate is 7:2.

[0061] Example 8 This embodiment uses most of the operating steps in Example 1 to prepare a fluorine-free positive electrode binder, a positive electrode sheet, and a lithium-ion battery. The difference lies in the amount of raw materials used in the preparation of the fluorine-free positive electrode binder: the mass ratio of natural cellulose to 4-cyano-4-((dodecylthiocarbonyl)thio)valerate is 100:7, the molar ratio of 4-cyano-4-((dodecylthiocarbonyl)thio)valerate to acrylonitrile and polyethylene glycol acrylate is 1:35, and the molar ratio of acrylonitrile to polyethylene glycol acrylate is 7:4.

[0062] Example 9 This embodiment uses most of the operating steps in Example 1 to prepare a fluorine-free positive electrode binder, a positive electrode sheet, and a lithium-ion battery. The difference lies in the amount of raw materials used in the preparation of the fluorine-free positive electrode binder: the mass ratio of natural cellulose to 4-cyano-4-((dodecylthiocarbonyl)thio)valerate is 100:2, the molar ratio of 4-cyano-4-((dodecylthiocarbonyl)thio)valerate to acrylonitrile and polyethylene glycol acrylate is 1:22, and the molar ratio of acrylonitrile to polyethylene glycol acrylate is 7:1.

[0063] Example 10 This embodiment uses most of the operating steps in Example 1 to prepare a fluorine-free positive electrode binder, a positive electrode sheet, and a lithium-ion battery. The difference lies in the amount of raw materials used in the preparation of the fluorine-free positive electrode binder: the mass ratio of natural cellulose to 4-cyano-4-((dodecylthiocarbonyl)thio)valerate is 100:8, the molar ratio of 4-cyano-4-((dodecylthiocarbonyl)thio)valerate to acrylonitrile and polyethylene glycol acrylate is 1:37, and the molar ratio of acrylonitrile to polyethylene glycol acrylate is 7:5.

[0064] Example 11 This embodiment uses most of the operating steps in Example 1 to prepare a fluorine-free positive electrode binder, a positive electrode sheet, and a lithium-ion secondary battery. The difference is that in the positive electrode sheet, the positive electrode active material is lithium nickel cobalt manganese oxide (NMC), and the mass ratio of the fluorine-free positive electrode binder to the positive electrode active material and conductive carbon black (Super P) is 2.5:95:2.5.

[0065] Example 12 This embodiment uses most of the operating steps in Example 1 to prepare a fluorine-free positive electrode binder, a positive electrode sheet, and a lithium-ion battery. The difference is that in the positive electrode sheet, the positive electrode active material is lithium nickel cobalt aluminum oxide (NCA), and the mass ratio of the fluorine-free positive electrode binder to the positive electrode active material and conductive carbon black (Super P) is 2.5:95:2.5.

[0066] Comparative Example 1 The positive electrode binder used in this comparative example is polyvinylidene fluoride (PVDF). Everything else is the same as in Example 1.

[0067] Comparative Example 2 This comparative example uses pure polyacrylonitrile (PAN) as the positive electrode binder. Everything else is the same as in Example 1.

[0068] Comparative Example 3 This comparative example uses a positive electrode binder obtained by physical blending cellulose and polymer. Specifically, chain transfer agent n-dodecyl mercaptan is added to NMP solvent, acrylonitrile and polyethylene glycol acrylate are mixed at a molar ratio of 7:3 and then added, followed by the initiator azobisisobutyronitrile (AIBN), with the amount of AIBN added being 1% of the total monomer mass. The mixture is stirred at 68°C for 9 hours under a nitrogen atmosphere to carry out the polymerization reaction, obtaining a polymer solution. Then, natural cellulose is added to the obtained polymer solution and physically blended to obtain the positive electrode binder. The mass ratio of natural cellulose to n-dodecyl mercaptan is 100:5, and the molar ratio of n-dodecyl mercaptan to the total molar ratio of acrylonitrile and polyethylene glycol acrylate is 1:30.

[0069] Performance testing: To better understand this application, the positive electrode binder, positive electrode sheet and lithium-ion battery prepared in the above embodiments and comparative examples were tested as follows, and the test results are shown in Table 1.

[0070] [Electrolyte Swelling Ratio]: Under standard atmospheric pressure and 60°C, 1M lithium hexafluorophosphate was added to a 1:1 mass ratio mixture of ethylene carbonate and methyl ethyl carbonate as the electrolyte. Two containers were taken and the electrolyte was added. The positive electrode binder was formed into a thin film, dried, and weighed to obtain W. 初始The film is a circular sheet with a diameter of 20 mm and a thickness of 20 μm. The positive electrode binder film is immersed in the electrolyte and left for 72 hours. After being removed and the electrolyte on the surface is wiped dry, the film is weighed to obtain W. 平衡 Calculate the electrolyte swelling ratio = (W 平衡 -W 初始 ) / W 初始 100%.

[0071]

Elongation at break

[0072] [Tensile Strength] The maximum stress that the positive electrode sheet can withstand during the tensile process is tested according to GB / T 1040.1-2018.

[0073] [Peel Strength]: The compacted positive electrode sheet was cut into 25 mm strips. High-strength 3M Scotch double-sided tape was used to firmly adhere the positive electrode coating side of the sample to a flat stainless steel plate, ensuring the aluminum foil current collector side faced upwards and could be freely clamped. On a universal testing machine, the aluminum foil was peeled from the coating at a 180° peel angle and a constant tensile speed. The average force required during peeling, divided by the sample width, is the peel strength of the positive electrode sheet, measured in N / m; it measures the degree of adhesion (bond strength) between the positive electrode coating and the current collector (aluminum foil).

[0074] [Capacity Retention] CR2032 coin cell lithium-ion batteries were assembled with each positive electrode and a lithium metal counter electrode, and charge-discharge cycle tests were conducted at a 1C rate: At 25°C, the batteries were charged at a constant current of 1C to the corresponding positive electrode material's charging cutoff voltage, such as: LFP: 3.65V; NMC523: 4.2V. Then, constant voltage charging was applied until the current dropped to 0.05C. Next, constant current discharge was performed at a 1C rate to the corresponding discharge cutoff voltage, such as: LFP: 2.5V; NMC523: 3.0V. This process was repeated 500 times. The discharge capacity Q1 at the first cycle and the discharge capacity Q at the 500th cycle were recorded. 500 ; Calculate the capacity retention rate Q = Q 500 / Q1 100%.

[0075] [Polarization Voltage at 5C Rate] Each positive electrode and lithium metal counter electrode were assembled into a CR2032 coin cell. Testing was conducted at a constant temperature of 25℃. Step 1: First, the battery was activated by two charge-discharge cycles at a 0.1C rate. Then, the following tests were performed: The battery was charged at a 1C rate with a constant current until the corresponding positive electrode material's charging cutoff voltage was reached (e.g., LFP: 3.65V, NCM523: 4.2V), then switched to constant voltage charging until the current ≤0.05C, and allowed to stand for 30 minutes; next, it was discharged at a 1C rate with a constant current until the corresponding discharge cutoff voltage was reached (e.g., LFP: 2.5V, NCM523: 3.0V), and the discharge capacity was recorded as C1. Step 2: The next cycle was immediately started: the battery was charged at a 1C rate to the same voltage and kept constant at ≤0.05C, allowed to stand for 30 minutes, and then discharged at a 5C rate with a constant current until the same cutoff voltage was reached. From the discharge curves of steps 1 and 2 respectively, take the voltage value corresponding to a discharge capacity of 50%C1, and denot it as V. 1C and V 5C According to the formula ΔV = V 1C - V 5C Calculate the polarization voltage at 5C rate; used to evaluate the rate performance of batteries, etc.

[0076] Table 1 Test Results The test results in Table 1 show that: The test results of Examples 1-12 and Comparative Examples 1-2 show that the fluorine-free positive electrode binder of this application has the lowest swelling rate in the electrolyte (less than 6%), and the best structural stability and electrolyte tolerance. When the fluorine-free positive electrode binder of this application is applied to the positive electrode sheet, the elongation at break can reach more than 235% (Example 1), which is 11.6 times that of PAN and 1.9 times that of PVDF, far superior to PVDF and PAN. It has excellent flexibility and can fully buffer the volume change of the positive electrode material. The tensile strength is between that of PVDF and PAN, indicating that the binder of this invention has sufficient mechanical support to prevent the electrode structure from collapsing, while avoiding the brittleness caused by excessive modulus (such as PAN), thereby synergistically improving flexibility and jointly ensuring the integrity of the electrode in long-term cycling. The peel strength can reach more than 36.5 N / m (Example 1), which is 2.0 times that of PVDF, and the interfacial adhesion is extremely strong. Comparative Example 1 (PVDF) showed a capacity retention of 87.7% after 500 1C cycles, with active material shedding occurring in the later stages of cycling. Comparative Example 2 (pure PAN) showed a capacity retention of 72.1% after 500 1C cycles, with electrode cracking after 100 cycles and a polarization voltage of 0.41V at 5C. In contrast, the present invention exhibits an average capacity retention of 93.7% after 500 1C cycles, reaching as high as 97%, and an average polarization voltage of 0.26V at 5C, less than 0.2V, demonstrating excellent cycle stability and high-rate performance during battery cycling. Therefore, the binder of this invention, when applied to cathode sheets and lithium-ion batteries, possesses fluorine-free and environmentally friendly characteristics and its overall performance far surpasses existing mainstream binders, making it compatible with mainstream cathode active material materials such as LFP, NMC, and NCA.

[0077] The test results from Examples 1-12 and Comparative Example 3 show that, compared to binders obtained through traditional physical blending of cellulose and polymers, the fluorine-free cathode binder of this application forms stable covalent bonds through the esterification reaction of a carboxyl-containing RAFT reagent with the hydroxyl groups of cellulose, replacing the traditional physical blending method. Simultaneously, combined with the precise grafting characteristics of RAFT controlled polymerization, it avoids side chain shedding and uneven distribution, improving the long-term stability of the network structure. Specifically, it exhibits lower swelling rate in the electrolyte (Example 2: 3.8% vs. Comparative Example 3: 7.2%), higher peel strength to the electrode (40.2 N / m vs. 26.5 N / m), and better capacity retention after 500 battery cycles (97.0% vs. 85.5%).

[0078] The test results of Examples 1 and 2 show that the addition of phosphate methacrylate monomer in the polymerization grafting reaction can form stronger coordination bonds with the surface of the cathode material, further improving the interfacial adhesion and electrolyte tolerance. At the same time, the phosphate ester group can help improve lithium-ion conductivity and optimize the high-rate performance of the battery.

[0079] The test results from Examples 1 and 7-10 show that when the mass ratio of linear polysaccharide and carboxyl-containing RAFT reagent, as well as the molar ratio between each raw material, are outside the range disclosed in this application, at least one of the following—bonding strength, flexibility, and rate performance—will decrease slightly: within the preferred ratio range, the bonding strength, flexibility, and rate performance are relatively better: when the mass ratio of linear polysaccharide to carboxyl-containing RAFT reagent is 100:5, the molar ratio of linear polysaccharide + RAFT reagent to acrylonitrile structural unit + acrylate structural unit is 1:30, and the molar ratio of acrylonitrile structural unit to acrylate structural unit is 7:3, the optimal ratio can achieve the best balance of bonding strength, flexibility, and rate performance while ensuring fluorine-free compliance, flexibility, and NMP solubility.

[0080] The present application has been further described above with reference to specific embodiments. However, it should be understood that the specific descriptions herein should not be construed as limiting the substance and scope of the present application. Various modifications made by those skilled in the art to the above embodiments after reading this specification are all within the scope of protection of the present application.

Claims

1. A fluorine-free positive electrode binder, characterized in that, The fluorine-free positive electrode binder includes copolymers. The copolymer comprises linear polysaccharide structural units, thiocarbonyl thio groups, acrylonitrile structural units, and acrylate structural units. The molar number of the thiocarbonyl thio groups is N1, and the total molar number of the acrylonitrile structural units and the acrylate structural units is N2. The ratio of N1 to N2 is 1:(25~35).

2. The fluorine-free positive electrode binder according to claim 1, characterized in that, The electrolyte swelling rate of the fluorine-free positive electrode binder is less than 6%.

3. The fluorine-free positive electrode binder according to claim 1, characterized in that, The ratio of the total mass of the linear polysaccharide structural unit to the total mass of the structural unit containing the thiocarbonyl thio group is 100:(3~7).

4. The fluorine-free positive electrode binder according to claim 1, characterized in that, The molar ratio of the acrylonitrile structural unit to the acrylate structural unit is 7:(2~4).

5. The fluorine-free positive electrode binder according to any one of claims 1-4, characterized in that, The linear polysaccharide structural unit is at least one of the following: natural cellulose structural unit, microcrystalline cellulose structural unit, nanocellulose structural unit, and chitin structural unit.

6. The fluorine-free positive electrode binder according to claim 5, characterized in that, The structural unit containing the thiocarbonyl thio group is derived from the RAFT reagent containing the carboxyl group.

7. The fluorine-free positive electrode binder according to claim 5, characterized in that, The acrylate structural unit is selected from at least one of polyethylene glycol acrylate structural units, methoxy polyethylene glycol acrylate structural units, methoxy polyethylene glycol methacrylate structural units, and ethylene glycol methyl ether acrylate structural units.

8. The fluorine-free positive electrode binder according to claim 5, characterized in that, The copolymer further includes phosphate ester structural units, wherein the molar ratio of the acrylonitrile structural units to the phosphate ester structural units is 7:(0.3~3).

9. A positive electrode plate, characterized in that, The positive electrode sheet includes the fluorine-free positive electrode binder as described in any one of claims 1 to 8.

10. A lithium-ion battery, characterized in that, The lithium-ion battery includes the fluorine-free positive electrode binder as described in any one of claims 1 to 8, or the positive electrode sheet as described in claim 9.

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